How to Access App_Code class in code behind file

 my purpose is to access the app_code class so that i cud pass the some control to it,,
look at my code
on button_click event i have passed the textbox2 id to app_code cs file where the class is FieldTestProcessor

FieldTestProcessor fs = new FieldTestProcessor();
FieldTestProcessor fs = new FieldTestProcessor();
fs.MyTextBox = TextBox2;

in my app_code file
i m accessiong it using
TextBox myText;  
public TextBox MyTextBox
{
get
{
return myText;
}
...